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HTML / CSS / JS

// Abstract class using Template Method pattern
abstract class DataProcessor {

// Template method: defines the algorithm skeleton
    public final void process(String data) {
        String validated = validate(data);      // step 1
        String transformed = transform(validated); // step 2 (abstract)
        store(transformed);                     // step 3 (abstract)
        System.out.println("Processing complete.");
    }

// Concrete shared step
    protected String validate(String data) {
        if (data == null || data.isBlank())
            throw new IllegalArgumentException("Data must not be blank");
        return data.trim();
    }

// Abstract steps — subclasses decide how to implement
    protected abstract String transform(String data);
    protected abstract void   store(String data);
}

class UpperCaseFileProcessor extends DataProcessor {
    @Override
    protected String transform(String data) {
        return data.toUpperCase();
    }

@Override
    protected void store(String data) {
        System.out.println("[FILE] Storing: " + data);
    }
}

class JsonProcessor extends DataProcessor {
    @Override
    protected String transform(String data) {
        return "{\"value\": \"" + data + "\"}";
    }

@Override
    protected void store(String data) {
        System.out.println("[DB] Inserting JSON: " + data);
    }
}

public class AbstractClassDemo {
    public static void main(String[] args) {
        // new DataProcessor();  // compile error — abstract class

DataProcessor file = new UpperCaseFileProcessor();
        file.process("  hello world  ");

System.out.println();

DataProcessor json = new JsonProcessor();
        json.process("  42  ");
    }
}